Thermostatic sensors of the present type are primarily designed for temperature-governing valves, e.g., radiator valves.
Several different embodiments of thermostatic sensors are already known, among others, sensors filled with gas, with liquid and with a wax-like filling of different compositions serving as an expansion medium.
The gas and liquid filled sensors contain elements such as ethyl ether, acetone or the like and in order to seal in their contents they have to be produced completely of metallic parts, which are soldered together. This is a very troublesome procedure, which raises the cost of manufacture considerably and nevertheless the result is a product which is sensitive to impacts, vibrations and similar disturbances, and which has a limited product life. Metallic bellows, for example not only are expensive but also must operate with a gas or a liquid which is of low viscosity and readily flows into and out of the crevices produced in the bellows. Furthermore metallic bellows tend to assume a permanent deformation. Apart from this, the handling of most of the gases and liquids in question present problems of hygiene and may be partially hazardous to health. The gas filled sensors moreover provide low adjusting power and limited stroke. The limited stroke is also a characteristic weakness of the liquid filled sensors.
In a sensor filled with wax of any type which produces a substantial stroke/.degree.C. gives entirely too great a hysteresis effect owing to substantial internal friction determined by the composition of the wax. Such a known wax-filled sensor consists of a smaller and shorter non-elastic metallic vessel, the edges of which are flanged outward in order to clamp around a rubber diaphragm, which seals off the wax filling in the vessel, as well as to clamp around the flange of a tubular member, which mounts a piston at its free, outer end. Between the piston and the diaphragm a free rubber plug is arranged to transfer the thermostatic movements of the wax filling from the diaphragm to the piston. This is a complicated construction, which is sensitive to disturbances, and which of course, like all other known sensor constructions, is expensive to manufacture.
With reference to their applications in use, it can be said that gas and liquid filled sensors can be mounted directly onto the valve which is to be governed, and are also designed for remote control via a capillary tube. Previously known wax-filled sensors are not suitable for remote control and are therefore only available for assembly directly on a valve.
The functional requirements, which a good sensor at present preferably must fulfill are the following: substantial adjusting power; low hysteresis; limited dead time, i.e., the time it takes from the moment when a change of temperature begins to the moment when the sensor begins to react to the changed temperature; suitable time constant, i.e., the time it takes from the moment when the sensor begins to react to the changed temperature to the moment when it has set itself at a given percentage of the final value, e.g., 63% (for prior art radiator valves the time constant usually is 20-30 minutes); suitable range of control (for radiator valves a suitable range is e.g., 8.degree.-26.degree. C.)); and substantial applicability for remote control.
Besides these enumerated functional requirements, manufacturing requirements are also made, such as methods of production which allow simple, inexpensive and rapid mass production, as well as use of, from the standpoint by hygiene and health, safe components.
The previously known sensors do not fulfill all of these requirements.